Mark Roberts

An event-related potential component sensitive to images of the human body.

One of the critical functions of vision is to provide information about other individuals. Neuroimaging experiments examining the cortical regions that analyze the appearance of other people have found partially overlapping networks that respond selectively to human faces and bodies. In event-related potential (ERP) studies, faces systematically elicit a negative component peaking 170 ms after presentation - the N170. To characterize the electrophysiological response to human bodies, we compared the ERPs elicited by faces, bodies and various control stimuli. In Experiment 1, a comparison of ERPs elicited by faces, bodies, objects and places showed that pictures of the human body (without the head) elicit a negative component peaking at 190 ms (an N190). While broadly similar to the N170, the N190 differs in both spatial distribution and amplitude from the N1 components elicited by faces, objects and scenes and peaks significantly later than the N170. The difference between N190 and N170 was further supported using topographic analyses of ERPs and source localization techniques. A unique, stable map topography was found to characterize human bodies between 130 and 230 ms. In Experiment 2, we tested the four conditions from Experiment 1, as well as intact and scrambled silhouettes and stick figures of the human body. We found that intact silhouettes and stick figures elicited significantly greater N190 amplitudes than their scrambled counterparts. Thus, the N190 generalizes to some degree to schematic depictions of the human form. Overall, our findings are consistent with intertwined, but functionally distinct, neural representations of the human face and body.

The selection and suppression of action: ERP correlates of executive control in humans.

A key aspect of higher cortical function is the ability to selectively withhold or suppress action where appropriate. To examine the time course of executive control we used dense-sensor EEG recording techniques to study event-related electrical potentials (ERPs) during a visual go/no-go task. We show that during both go and no-go trials there is a positive deflection in the ERP, which develops over posterior parietal sensors approximately 350 ms (P300) after the onset of a conditional visual stimulus, but is selectively suppressed during no-go trials. We also show that this modulation of the parietal P300 is preceded by a negative deflection in the ERP recorded over frontal cortex (N2), which is apparent only for no-go trials. We suggest that this signal provides an electrophysiological marker in man for the decision to withhold the execution of a motor response.

Familiar words capture the attention of 11-month-olds in less than 250 ms.

The capacity of human infants to discriminate contrasting speech sounds specializes to the native language by the end of the first year of life, when the first signs of word recognition have also been found, using behavioural measures. The extent of voluntary attentional involvement in such word recognition has not been explored, however, nor do we know what its neural time-course may be. Here we demonstrate that 11-month-old children shift their attention automatically to familiar words within 250u2009ms of presentation onset by measuring event-related potentials elicited by familiar and unfamiliar words. A significant modulation of the first negative peak (N200), known to index implicit change detection in adults, was induced by word familiarity in the infants.

Glutamatergic correlates of gamma-band oscillatory activity during cognition: A concurrent ER-MRS and EEG study

Frequency specific synchronisation of neuronal firing within the gamma-band (30-70 Hz) appears to be a fundamental correlate of both basic sensory and higher cognitive processing. In-vitro studies suggest that the neurochemical basis of gamma-band oscillatory activity is based on interactions between excitatory (i.e. glutamate) and inhibitory (i.e. GABA) neurotransmitter concentrations. However, the nature of the relationship between excitatory neurotransmitter concentration and changes in gamma band activity in humans remains undetermined. Here, we examine the links between dynamic glutamate concentration and the formation of functional gamma-band oscillatory networks. Using concurrently acquired event-related magnetic resonance spectroscopy and electroencephalography, during a repetition-priming paradigm, we demonstrate an interaction between stimulus type (object vs. abstract pictures) and repetition in evoked gamma-band oscillatory activity, and find that glutamate levels within the lateral occipital cortex, differ in response to these distinct stimulus categories. Importantly, we show that dynamic glutamate levels are related to the amplitude of stimulus evoked gamma-band (but not to beta, alpha or theta or ERP) activity. These results highlight the specific connection between excitatory neurotransmitter concentration and amplitude of oscillatory response, providing a novel insight into the relationship between the neurochemical and neurophysiological processes underlying cognition.

ERP evidence for the split fovea theory.

According to the bilateral representation theory, a complete copy of the words presented foveally is received simultaneously in the left and right visual cortices. However, a growing body of observations, which has led to the split fovea theory, proposes a functional split of the foveal area between the two hemispheres. In the present study we tested these two accounts using an adapted version of the Reicher-Wheeler paradigm. Ten control participants and ten participants with developmental dyslexia undergoing electroencephalographic recordings were asked to identify one of five letters in a string. The target letter was systematically presented at fixation but the horizontal positioning of the letter string was varied such that the stimulus fluctuated in both the visual hemifields over the experiment. ERP results showed that letter strings encompassing the foveal field were not sent to both cerebral hemispheres simultaneously when fixation coincided with extreme letter positions (i.e., first or last). Indeed, the P1 peak was delayed in this case, which was interpreted as the result of a transfer of visual information from the contralateral hemisphere via the splenium of the corpus callosum. Consistent with the split fovea theory, this result suggests that a minimal amount of graphic input is necessary to induce a P1 event. The interhemispheric transfer time (IHTT) deducted from peak-to-peak P1 latency delays ranged from 26 to 42 ms. As previously observed, the IHTT was significantly faster for right-to-left than left-to-right transfer in the control group. IHTT was marginally shorter in control participants as compared to participants with developmental dyslexia, and the faster transfer to the left hemisphere seen in the former was not found in the latter.

Functional characterisation of the extrastriate body area based on the N1 ERP component

Electrophysiological and functional neuroimaging evidence points to the existence of neural populations that respond strongly and selectively to the appearance of the human body and its parts. However, the relationship between ERP and fMRI markers of these populations remains unclear. Here we used a previously identified functional dissociation between two body-selective regions identified with fMRI (extrastriate body area or EBA; fusiform body area or FBA) in order to better understand the source of a body-selective N1 ERP component. Specifically, we compared the magnitude of the N1 elicited by images of fingers, hands, arms and bodies to that obtained for hierarchically-matched control stimuli. We found close agreement between the pattern of body-part selectivity exhibited by N1, and the pattern of BOLD selectivity elicited in a previous study by the same type of stimuli in EBA (in contrast to FBA). We interpret these findings as evidence for EBA as the primary generator of the body-selective N1 component. Our results are an example of the use of functional criteria to distinguish among the possible neural sources of ERP markers.

Brain Potentials Dissociate Emotional and Conceptual Cross-Modal Priming of Environmental Sounds

The attentional effects triggered by emotional stimuli in humans have been substantially investigated, but little is known about the impact of affective valence on the processing of meaning. Here, we used a cross-modal priming paradigm involving visually presented adjective-noun dyads and environmental sounds of controlled affective valence to test the contributions of conceptual relatedness and emotional congruence to priming. Participants undergoing event-related potential recording indicated whether target environmental sounds were related in meaning to adjective-noun dyads presented as primes. We tested spontaneous emotional priming by manipulating the congruence between the affective valence of the adjective in the prime and that of the sound. While the N400 was significantly reduced in amplitude by both conceptual relatedness and emotional congruence, there was no interaction between the 2 factors. The same pattern of results was found when participants judged the emotional congruence between environmental sounds and adjective-noun dyads. These results support the hypothesis that conceptual and emotional processes are functionally independent regardless of the specific cognitive focus of the comprehender.

Similarity, not complexity, determines visual working memory performance

A number of studies have shown that visual working memory (WM) is poorer for complex versus simple items, traditionally accounted for by higher information load placing greater demands on encoding and storage capacity limits. Other research suggests that it may not be complexity that determines WM performance per se, but rather increased perceptual similarity between complex items as a result of a large amount of overlapping information. Increased similarity is thought to lead to greater comparison errors between items encoded into WM and the test item(s) presented at retrieval. However, previous studies have used different object categories to manipulate complexity and similarity, raising questions as to whether these effects are simply due to cross-category differences. For the first time, here the relationship between complexity and similarity in WM using the same stimulus category (abstract polygons) are investigated. The authors used a delayed discrimination task to measure WM for 1-4 complex versus simple simultaneously presented items and manipulated the similarity between the single test item at retrieval and the sample items at encoding. WM was poorer for complex than simple items only when the test item was similar to 1 of the encoding items, and not when it was dissimilar or identical. The results provide clear support for reinterpretation of the complexity effect in WM as a similarity effect and highlight the importance of the retrieval stage in governing WM performance. The authors discuss how these findings can be reconciled with current models of WM capacity limits.

The late positive potential indexes a role for emotion during learning of trust from eye-gaze cues

Gaze direction perception triggers rapid visuospatial orienting to the location observed by others. When this is congruent with the location of a target, reaction times are faster than when incongruent. Functional magnetic resonance imaging studies suggest that the non-joint attention induced by incongruent cues are experienced as more emotionally negative and this could relate to less favorable trust judgments of the faces when gaze-cues are contingent with identity. Here, we provide further support for these findings using time-resolved event-related potentials. In addition to replicating the effects of identity-contingent gaze-cues on reaction times and trust judgments, we discovered that the emotion-related late positive potential increased across blocks to incongruent compared to congruent faces before, during and after the gaze-cue, suggesting both learning and retrieval of emotion states associated with the face. We also discovered that the face-recognition-related N250 component appeared to localize to sources in anterior temporal areas. Our findings provide unique electrophysiological evidence for the role of emotion in learning trust from gaze-cues, suggesting that the retrieval of face evaluations during interaction may take around 1000 ms and that the N250 originates from anterior temporal face patches.

The time course of activation of object shape and shape+colour representations during memory retrieval.

Little is known about the timing of activating memory for objects and their associated perceptual properties, such as colour, and yet this is important for theories of human cognition. We investigated the time course associated with early cognitive processes related to the activation of object shape and object shape+colour representations respectively, during memory retrieval as assessed by repetition priming in an event-related potential (ERP) study. The main findings were as follows: (1) we identified a unique early modulation of mean ERP amplitude during the N1 that was associated with the activation of object shape independently of colour; (2) we also found a subsequent early P2 modulation of mean amplitude over the same electrode clusters associated with the activation of object shape+colour representations; (3) these findings were apparent across both familiar (i.e., correctly coloured – yellow banana) and novel (i.e., incorrectly coloured - blue strawberry) objects; and (4) neither of the modulations of mean ERP amplitude were evident during the P3. Together the findings delineate the timing of object shape and colour memory systems and support the notion that perceptual representations of object shape mediate the retrieval of temporary shape+colour representations for familiar and novel objects.